1. Field of the Invention
The present invention is generally related to transition metal carbene complexes and the luminescent application thereof, and more particularly to transition metal biscarbene complexes and the electroluminescent application thereof.
2. Description of the Prior Art
In the previous reports, most researchers relied on the modification of C,N-heteroaromatic (C^N) ligand for wide-range altering of the emission wavelength and color. For example, fac-Ir(ppy)3 (fac-tris(2-phenylpyridine)iridium) is well known to give green light emission. By increasing the conjugation of the ligand to 1-phenylisoquinoline in Ir(piq)3 (tris[1-phenylisoquinolinato-C2,N]iridium), the emission of the iridium complex shifts from green to deep red. Other effective approaches for tuning the emission energy of cyclometalated iridium complexes include the introduction of electron withdrawing or donating groups to the aryl ring or pyridine rings and the use of different third ligand. The well-known light-blue iridium complex, Flrpic (iridium(III) bis(4,6-difluorophenylpyridinato-N,C2′)picolinate) was designed based on the green emission Ir(ppy)2(acac) (bis(2-phenylpyridine)(acetylacetonate)iridium(III)) by the introduction of electron-withdrawing fluoro groups to the phenyl ring and the use of pic ligand as the third ligand. A few other blue iridium complexes containing two 4,6-difluorophenylpyridinato-N,C2′ ligands were also known to use various third ligands for the fine tune of the emission colors. Recently, Lee et al. synthesized a deeper blue dopant, FCNIr (tris((3,5-difluoro-4-cyanophenyl)pyridine)iridium) by the introduction of an electron withdrawing CN group to the 3,5-difluorophenyl)pyridine ligand. A device based on this complex as the dopant emitter showed high external quantum efficiency of 18.4% and CIEx+y (Commission International de l'Eclairage)<0.30.
Iridium tris(carbene) complexes are known to have high triplet energy gaps and can be used as blue phosphorescent emitters. Kido et al. have reported a triscarbene iridium complex-based device having a high external quantum efficiency of 18.6% with CIE coordinate of (0.15, 0.19).
In the prior art, it is usually a hard trial to employ different corn structures and try to put different functional group thereon to form a new complex for providing different emitting color. Because it is a long and difficult process for synthesis a new proper luminescent material, it is a hard and expensive work to change the emitting color.
In view of the above matter, developing a novel transition metal carbene complex and device thereof having high luminance efficiency and wide-range color tuning is still an important task for the industry.